Defining a fallback for model

[CSenshi]

Package has feature to define fallback url or path which works great when getFirstMediaUrl or getFirstMediaPath return null.

However while using responsive images in blade.php we can't really use this feature. because getFirstMedia() still returns null. Maybe return new model instance with given fallback url/path? That would help getting rid of if/else in blade.php files

Example:

 {{ $blog->getFirstMedia() }} // <-- returns null, nothing 

 {{ $blog->getFirstMedia()->img('', ['class' => 'post-img']) }}  // <-- returns error, Call to a member function img() on null